The instant invention relates to a timing belt or cable, and more particularly to an inelastic timing belt or cable driven by a rigid swinging arm.
In a mechanism wherein a timing belt (defined for purposes of this specification and claims appended thereto to include a cable) mounted on conventional pulleys is driven by a rigid swinging arm attached at one end to the timing belt, tension (or slack) in the timing belt changes because the arm tip (point of connection to the belt) does not follow a theoretical elliptical path which would maintain a constant tension in the belt but rather traces the arc of a circle which arc is transmitted to the straight section of the belt. As the arm tip swings from a position near one pulley to the midpoint of the belt section equidistant from the two pulley centers, tension in the timing belt increases from a minimum to a maximum.
The variation in the tension (or slack) in the timing belt prevents precise motion and accurate positioning of the components driven by the belt, beginning with the driven pulley. This desirable and simple system utilizing one belt and a rigid swinging arm is thereby rendered unusable in many applications. An elastic belt, sized to provide minimum tension at the start and finish of motion, stretches at mid-motion producing excessive friction loads in the pulley bearings. Use of an elastic belt also causes errors in positioning the driven components in proportion to the magnitude of the driven load. Thus, an elastic belt provides no real answer to the problem of the variable tension in the belt.
An inelastic belt, sized to provide minimum tension at mid-motion will have excessive slack at the start and finish of motion and will fail to provide precise positioning of the driven components. The instant invention accordingly overcomes the problem of excessive slack at the start and finish of motion when an inelastic belt is used by providing continuous compensation at negligible increase in cost and complexity of parts. The continuous compensation is achieved by using a pulley so designed to remove slack from an inelastic belt at both ends of the travel and to restore slack at the mid-point of the motion. The effect is virtually identical to that obtained by a change in the center distance between conventional pulleys by an amount necessary to take up the slack in an inelastic belt introduced by a swing from mid-motion to either end.